Study On The Precise Differential Positioning Model Based On The Multi-frequency And Multi-system GNSS

At present,Global Navigation Satellite System(GNSS)includes GPS of the United States,GLONASS of Russia,Bei Dou(BDS)of China,and Galileo of the European Union.With the advancement and continuous improvement of multiple GNSS construction,three or more frequencies will be used in each system,the satellite navigation and positioning has entered an era of coexistence and interoperability of multiple systems and multiple frequencies.Therefore,it is necessary to improve the existing precision positioning technology based on the multi-system and multifrequency observations,so that the further demands of GNSS precise applications can be met better.Based on these backgrounds,this thesis is intended to study the key technologies in GNSS high-precision and real-time relative positioning with the fusion of multisystem and multi-frequency observations,which mainly involves: the processing strategies of positioning bias of different frequency and different system observations,differential inter-system bias(DISB)and differential inter-frequency bias(DIFB)to improve the applicability of the model,the consistency of the solution results,and the accuracy of positioning.The main work and contributions of this thesis are listed as follows:1.A new method of RTK suitable for baselines of different lengths is developed.A new method of RTK is developed which is suitable for baselines of different lengths.In the first step,the double-differenced wide-lane ambiguities were calculated and fixed using the pseudo-range and carrier-phase wide-lane combination observations.Once the double-differenced wide-lane integer ambiguities were fixed,the wide-lane combined observations were regarded as accurate pseudo-range observations.Subsequently,the carrier-phase wide-lane,and ionosphere-free combined observations were used to fix the double-differenced carrier-phase integer ambiguities,achieving the final RTK positioning.The results show that in the case of a short baseline,the highprecision wide-lane phase combination observations and carrier phase ionospheric observations combination(LWLC)method and the single frequency pseudo-range and carrier phase observations combination(P1L1)have the same positioning accuracy,while the positioning results of the combination of pseudo-range and carrier phase ionosphere-free observations combination(PCLC)are relatively poor;in the case of medium and long baselines,the positioning accuracy of the LWLC method was slightly higher than that of the PCLC method,but it is significantly better than the P1L1 method.These conclusions comprehensively show that the LWLC method can provided high-precision RTK positioning results for baselines with different lengths,as it used high-precision carrier-phase observations with fixed ambiguities instead of lowprecision pseudo-range observations.2.The processing models of the positioning bias of the different systems and different frequencies observations are established.(1)The properties of positioning bias of different systems observations in relative positioning are analyzed in detail,and the direct difference method and real-time estimation model are proposed.The results show that: the positioning bias of different system observations obtained by different baselines are different;the positioning bias of different system observations obtained by the same baseline and different days show consistency and periodicity;the direct difference method and parameter estimation model of the positioning bias of different system observations can obtain the consistent positioning results of single system;the direct difference method and parameter estimation model of positioning bias of different system observations can obtain the same positioning results of single system and the positioning bias of different system observations obtained in the same time period for many consecutive days are relatively stable.(2)The properties of positioning bias of different frequency observations in relative positioning are analyzed in detail,and the direct difference method and realtime estimation model are proposed.The results showed that there were mm-cm differences in the positioning results of different frequencies observed in RTK.The positioning deviation between GPS L1 and L2 observation values of different frequencies is smaller and more stable than that between BDS B1 and B2.Both the direct difference method and the parameter estimation model can obtain the positioning results consistent with that of a single frequency.The positioning deviation of different frequency observations obtained by direct difference method and parameter estimation model remains consistent and relatively stable in the same time period for several consecutive days.3.The processing models of differential inter-system biases and differential inter-frequency biases are presented.(1)The characteristics of differential inter-system bias(DISB)are analyzed in detail,and the correction model and real-time estimation model of DISB are proposed.In the inter-system differencing model for the GPS and BDS combined RTK,the pseudo-range and carrier phase DISB differed for varying frequencies(L1–B1,L2–B2),but the pseudo-range and carrier phase DISB remain stable during a continuous observation period.The pseudo-range DISB amplitudes relative to the mean value were all within 1 m,and the STD was less than 0.45 m,while the carrier phase DISB amplitudes for both L1–B1 and L2–B2 relative to the mean value were within 0.05 cycles,and the STD was less than 0.01 cycles.Please note that these values apply to the five-day period of analysis.When the baseline receiver types were the same,the pseudo-range DISB was generally small;when the baseline receiver types differed,the pseudo-range DISB was more significant,and could reach several meters.The DISB correction model and the real-time estimation model can improve the consistency of the positioning results,and the positioning accuracy of the inter-system differencing model was improved by approximately 30% compared to that of the classical intrasystem differencing model.(2)The characteristics of differential inter-frequency bias(DIFB)are analyzed in detail,and the correction model and real-time estimation model of DIFB are proposed.The results show that in the GPS and BDS inter-frequency differencing model,the pseudo-range and carrier phase DIFB corresponding to GPS L1-L2 and BDS B1-B2 are different,but in a continuous observation period,the pseudo-range and carrier phase DIFB are relatively stable.The pseudo-range DIFB amplitudes relative to the mean value were all within 2 m,and the STD was less than 0.5 m,while the carrier phase DIFB amplitudes for both the GPS L1–L2 and BDS B1–B2 relative to the mean value were within 0.1 cycles,and the STD was less than 0.015 cycles.The DIFB correction model and the real-time estimation model can improve the consistency of the positioning results,and the positioning accuracy of the inter-frequency differencing model was improved by more than 30% compared to that of the classical intrafrequency differencing model.4.The positioning performance of the new BDS-3 signal of double-differenced network RTK(NRTK)and undifferenced network RTK(URTK)is analyzed.According to the pseudo-range and carrier phase observations of the new signal of BDS-3 received by the regional reference station network,the correction of doubledifferenced(DD)atmospheric delay with centimeter precision can be obtained.In NRTK,using the DD ionospheric delay and tropospheric delay corrections obtained by reference station network,the DD atmospheric delay corrections of virtual reference station near the rover station are generated,and then the ultra-short baseline RTK positioning solution is formed with the rover station to achieve the centimeter level positioning of the rover station.In URTK,the undifferenced observations of rover station is corrected by using the undifferenced corrections interpolated by the rover station's approximate coordinates,and then the instantaneous ambiguity is fixed by PPP mode,and the centimeter level positioning can also be achieved.With the completion of BDS-3 and the formal opening of services,it will help to achieve more accurate and stable real-time kinematic positioning services by integrating BDS with other GNSS.